Power amplifiers have been widely used as a final stage in multistage amplifiers, such as in audio amplifiers and radio transmitters, to deliver maximum power to a load, rather than maximum voltage gain, for a given percentage of distortion. One particular application of a power amplifier is in a disk drive device. In essence, a power amplifier acts as a transconductance amplifier and takes an input voltage and converts it into an output current to drive a voice coil motor (VCM).
FIG. 1, a disk drive 10 is shown, which contains one or more magnetic disks 12 that spin about a spindle 14. Data is written with onto disks 12 by a read/write head 16, and information is read back using the same read/write head 16. Read/write head 16 is attached to the bottom of a suspension 18 which is part of an actuator 20 that rotates about a pivot point 22. Actuator 20 is moved by current through a voice-coil motor 24. Crash stops (26 and 28) limit the travel of actuator 20. FIG. 2 shows a first close-up view of a load and unload ramp. FIG. 3 shows a second close-up view of the load and unload ramp.
A close-up view of load/unload ramp 30 is shown in FIGS. 2 and 3. While only a single disk and two heads are shown, multiple disks and multiple sets of heads are also possible. Disk 12 is mounted on spindle 22 and is rotated by a spindle motor 40. Read/write heads 16 are attached to suspensions 18 that contain tabs 32. Before disk 12 is stopped, tabs 32 are pushed onto load/unload ramp 30 which is attached to base plate 42 of disk drive 10 by a mounting screw 44 or other fastening means. Heads 16 are lifted off disk 12 by tab 32 on the end of suspension 18 that travels up load/unload ramp 30.
FIG. 3 is a top view of the arrangement shown from the side in FIG. 2 and illustrates the direction 46 which tab 32 moves when the tip of suspension 18 is pushed onto load/unload ramp 30 before disk 12 is stopped.
FIG. 4 illustrates a portion of a disk drive device controller, e.g., a load/unload controller, that illustrates a power amplifier 110 coupled to a load comprising a sensing resistor 112, Rs, and a VCM 114. When a control output signal having a zero voltage value is transmitted to the power amplifier 110 after conversion to analog form through a digital-to-analog (D/A) converter 116, an output current from the power amplifier may be non-zero due to the intrinsic offsets of the components forming the power amplifier 110. The non-zero output current is generally known as the output offset current.
The problem with the output offset current is that it can be large in high-performance disk drives due to the low resistance of the current sensing resistor 112. For example, a power amplifier 110, whose current sensing amplifier 118 has 5 mV (millivolts) of input offset voltage and uses a 200 milliohm sensing resistor 112, can have an output offset current on the order of 25 miilliamperes. In a high performance disk drive with relatively large torque constants, an offset current of that value is enough to generate sufficient torque to move the actuator of the disk drive, which can be a problem in drives with a ramp for head loading, if the force of the heads on the ramp and the ramp's coefficient of friction are low enough to be overcome by this torque. This is especially true if the actuator is not normally mechanically latched when the heads are in the unloaded position. The result could be accidental head loading or unloading due to the output offset current alone if it is not sufficiently reduced or eliminated.
Accordingly, what is needed is a system and method for nulling the output offset current in a power amplifier. The present invention addresses such a need.